Oxygen metabolism and resultant oxidative damage caused by metabolically generated reactive oxygen species (ROS) are key processes affecting the activities of plaque bacteria. The organisms have developed multiple mechanisms to protect themselves against oxidative damage, including superoxide dismutase, peroxidases, sulfhydryl reductases and DNA repair enzymes. The major source of O2 for supragingival plaque is air in the mouth, while that for subgingival plaque is crevicular fluid. Since plaque is a thin biofilm, O2 moves readily into the film to be metabolized, and plaque generally has been found to have residual O2 levels about 10% that of air-saturated water. However, the important O2 is the metabolized portion, which then is the source of ROS. In addition, many oral healthcare products contain hydrogen peroxide, which can be radicalized through Fenton reactions to cause oxidative damage. The objectives of this application are to develop a clearer picture of oxidative damage to plaque bacteria, primarily oral streptococci, and to identify through use of genetic mutants the major protective mechanisms. Part of the project has a very practical aim of devising better ways to control oral infectious diseases with an emphasis on caries and use of peroxides. The specific aims are: 1. Determine the roles played by NADH-linked oxidases, peroxidases and glutathione reductase in protection of oral streptococci against hydrogen peroxide and other oxidatively damaging agents, 2. Define the roles that superoxide and superoxide dismutase play in oxidative damage and protection against damage, 3. Elucidate the interactions of transition-metal cations, chelators and fluoride in oxidative damage, including interactions with oxygen- metabolizing and protective enzymes.